The present invention relates to an expansion member for use on a paper making machine and particularly located at the breast box of the machine.
A paper making machine has a breast box including an outlet slot from which the fiber suspension is sprayed. The outlet slot extends over the cross machine width of the machine. To adjust the height of the outlet slot between the two lips of the slot, an expansion chamber is disposed on at least one lip, and controlled expansion of the chamber bends the lip and determines the slot height. There are also other applications for an expansion chamber in a paper making machine.
Such an expansion member is known from Federal Republic of Germany Published Application DE 37 28 387 A 1, equivalent to U.S. Pat. No. 4,867,847. In that case, the expansion member is essentially a hollow beam including pressurizable, expansible cable. The beam includes a plurality of separate pressurizable cells over its length in the cross machine direction. The cells, which can be individually acted on by pressure, have a box profile, as seen in a cross section transverse to the machine direction and perpendicular to the fiber web. The partition walls between two adjacent cells are highly stretchable, and these partition walls can increase in length upon a corresponding change of the internal pressure within one cell, as compared with the pressure in the neighboring cell. The side partition walls are joined by an upper transverse wall above the cell and a lower transverse wall below the cell. The upper transverse wall rests against a firm surface. The lower transverse wall, which is effectively the bottom of the cell, must move perpendicular to the fiber web moving out of the outlet slot of the breast box. If the expansion member is placed in the vicinity of the outlet slot or outlet channel from the breast box, then the slot or channel height or cross section is narrowed by movement of the bottom of the cell in the direction toward the fiber web passing out the outlet slot.
The purpose of the above described known device is to act directly on the weight per unit area profile of the material moving out the slot or channel at very specific points transverse to the machine direction. To do this, individual cells are acted on with pressures of different value. It has been found that it is not possible precisely to locally limit the influence on the height of the outlet slot and thus the weight per unit area, as would be desired. Instead, the pressure which is applied to a single cell acts, as seen transversely to the machine direction, also beyond the length of that cell, i.e. it also acts on the neighboring cells. This may be undesirable in a specific case.
The inventor here has found the following to be the cause of this problem. If a cell in a coherent box profile is acted on by pressure, then not only do the side or partition walls which define this cell lengthen, but the adjacent partition walls of the neighboring cells are also affected to a greater or lesser extent and thus they are also lengthened, so that a change in cross section of the outlet channel, which can be delimited precisely in zonewise fashion with respect to each cell, is not possible.